Liposomes are completely closed lipid bilayer membranes containing an entrapped aqueous volume. Liposomes may be unilamellar vesicles (possessing a single membrane bilayer) or multilamellar vesicles (onion-like structures characterized by multiple membrane bilayers, each separated from the next by an aqueous layer). The bilayer is composed of two lipid monolayers having a hydrophobic "tail" region and a hydrophilic "head" region. The structure of the membrane bilayer is such that the hydrophobic (non-polar) "tails" of the lipid monolayers orient toward the center of the bilayer while the hydrophilic (polar) "heads" orient toward the aqueous phase. Liposomes are useful for delivering bioactive agents such as drugs to mammals, including humans.
Generally, a principal ingredient of the lipid bilayers are phospholipids such as phosphatidylcholine. The resulting liposomes are often unstable and degrade over time. This instability may make the pharmaceutical use and commercial development of liposomal drug delivery systems less attractive because of the reduced shelf-life of the products or the need to take special care in storing the products.
The analysis of lipid composition may be performed by a variety of methods. Among those methods are high performance liquid chromatography using ultraviolet detection at 205 nm ("UV at 205 nm"); thin layer chromatography of phospholipids using a phosphorus specific assay based on the zinzadze reagent (such as Phospray.RTM., Supelco Co. ) ("TLC"); high pressure liquid chromatography using flame ionization; and thin layer chromatography using flame ionization (e.g., Iatroscan (TH-10).TM., Iatron Laboratories, Inc., Japan). Each method will yield results peculiar to the test method. The lipid compositions of this invention have been analysed by both UV at 205 nm and TLC. The UV at 205 nm test method utilized herein is accomplished by high performance liquid chromotagraphy on a silica gel column. The TLC test method as utilized herein is accomplished by spotting about a 200 ug sample of egg phosphatidylcholine ("EPC") on a silica gel plate (such as HPTLC-HL preadsorbant plate, Analtech, Inc.). UV at 205 nm as compared to TLC tests of identical materials produced slightly higher apparent purity for the UV at 205 nm tests.
Purities noted herein will be expressed based upon TLC analysis unless otherwise stated.